I am currently exploring a similar problem (Debian wheezy/sid on an
up-to-date 64amd kernel with KDE and a combined front/back end myth
0.24.1+ on a homebuilt Gigabyte AMD based MB using onboard 5.1
sound/video-HD4200. HDMI output to the TV with sound to the receiver
via an optical cable).
My OS sound stopped working several months(?) ago, and I too did not
notice right away. In my case, I suspect a general system upgrade
in mid-summer when other software configurations were also changed by
the upgrade. I currently am focused on looking at the configuration
of pulseaudio (with phonon/alsa/jack/kmix, etc. thrown into the mix)
On Myth, sound through pulseaudio overall works well, although there
are some ocassional changes in quality after a commercial jump. The
FE does report it releases the audio connection when it shuts down.
However, without the backend running the OS still does not give sound,
nor does flash content in a browser. ( I have tried both the non-free
and Debian flash software with no success, so I think the problem is
elsewhere, despite all I see on the web about issues with flash.)
Hopefully over the next couple of weeks I will get enough time to sort
this out. Among the links I have found most helpful are those on
wiki.archlinux.org, and specifically
https://wiki.archlinux.org/index.php/PulseAudio#KDE_4_and_Qt4
Good luck
Wayne